Method for mixing two initial solutions

ABSTRACT

In a process for mixing two initial solutions to produce a working solution with an accurately defined mixing ratio, the two initial solutions are coarsely mixed in a first step, having a mixing ratio lying within a given range, and the value of an internal parameter of the working solution is determined in a second step. The values of this internal parameter are known and differ significantly for the two initial solutions. Then the accurate mixing ratio of the working solution is determined from the measured value of the internal parameter in a third step.

BACKGROUND OF THE INVENTION

The invention relates to a method for mixing two initial solutions toproduce a working solution with an accurately known mixing ratio, and acalibrating device for implementing this method.

Accurate mixtures of liquids are often produced with the use ofexpensive dilutor systems. Such systems permit the required mixingratios to be obtained in an exactly reproducible manner.

In various applications, however, the required mixtures need not have anaccurately defined mixing ratio; sometimes exact knowledge of the actualmixing ratio obtained will suffice. In those instances the use ofexpensive mixing systems is superfluous.

DESCRIPTION OF THE PRIOR ART

An improvement of known systems is presented in AT-B 392 364, whichdescribes a method for calibrating a measuring device and an apparatusfor implementation of the method. In order to calibrate a measuringdevice, which is used for determining at least the pH and pCO₂ values inaqueous solutions, it is proposed that two aqueous parent solutions ofgood storage stability, A and B, be mixed together at a defined ratiojust before calibration, and that the desired pH and pCO₂ values be madeavailable for calibration of the respective measuring electrodes of thedevice only after chemical reaction of the parent solutions A and B hasoccurred. To determine the accurate mixing ratio it is suggested that acoloring agent be added to one of the parent solutions A or B, and thatthe mixing ratio of the working solution be determined by means ofoptical methods, for example, absorption measurements. The document alsoindicates that it would be possible to add other chemical or physicalmarkers to at least one of the parent solutions, for instancefluorescence quenchers, or radioactively tagged substances.

This method suffers from the drawback that certain substances must beadded to one of the parent solutions. In addition to making the processmore complicated, this measure also has the effect that the substancesadded to the parent solution will then be contained in the workingsolution where they might interfere with the planned use of thesolution.

SUMMARY OF THE INVENTION

It is an object of the invention to improve a mixing process asdescribed above in such a way as to ensure simplicity of use whilelargely avoiding contaminants in the working solution.

In the invention this object is achieved by coarsely mixing the twoinitial solutions in a first step, the mixing ratio lying within a givenrange, and by determining the value of an internal parameter of theworking solution in a second step, the values of this parameter beingknown and differing significantly for the two initial solutions, and bydetermining the exact mixing ratio of the working solution from themeasured value of the internal parameter in a third step. Thefundamental idea of the invention is that the mixing ratio should bedetermined with the use of a variable or parameter which is a prioripresent or inherent in the initial solutions, or at least in one ofthese solutions. In this way the new method will make superfluous theuse of expensive precision equipment for mixing the solutions and theaddition of markers.

The method of the invention essentially comprises two processes: coarsemixing of the initial solutions and precise measuring of the mixingratio with the use of an internal parameter.

Particularly good results are obtained if the value measured for theinternal parameter is employed as a controlling or adjusting variablefor correcting the mixing ratio during coarse mixing, as is proposed infurther development of the invention. As a consequence, the mixing ratiomay be controlled or adjusted by a feedback of the internal parameter.

In further development of the invention it is proposed that forcalibrating purposes the working solution be a calibrating solution withat least one calibrating parameter, the exact value of the calibratingparameter being calculated from the measured value of the internalparameter, and at least one variable from the group of pH value and CO₂partial pressure of the calibrating solution being used as calibratingparameter.

The method of the invention is particularly useful in the calibrating ofion-selective electrodes by providing that at least one ionicconcentration of the calibrating solution be employed as calibratingparameter, i.e., preferably the concentration of Na⁺, K⁺, Li⁺, Ca⁺⁺,Mg⁺⁺, Cl⁻ or HCO₃ ⁻ ions.

It is further provided for the calibration of biosensors that at leastone enzyme substrate concentration of the calibrating solution beemployed as calibrating parameter, i.e., preferably the concentration ofglucose, lactate, urea, or creatinine.

In the invention the internal parameter to be measured may beconductivity, electrical resistance, optical transmission, opticalabsorption, or intrinsic luminescence of the working solution.Conductivity is measured without the use of special markers, i.e., noconductive additive is required for the solutions. As all organic buffersystems exhibit intrinsic luminescence, for instance, luminescence maybe measured in application of the invention.

In the instance of ions and CO₂ and pH, the relationship betweenconductivity and the calibrating parameter is trivial, as all ionscontribute towards conductivity. With enzyme substrates a correlationwith conductivity is established, for example, if the calibratingsolutions are used in buffered pH environment, and if each pH buffersystem is conductive on account of the dissociation equilibria.

If conductivity is used as an internal parameter the method of theinvention may be employed for all calibrating solutions with a pH buffersystem. The two buffer components, buffer acid and buffer base are usedseparately in the two initial solutions, for example as described inAT-B 392 364.

It is also possible to combine the above calibrating parameters asrequired, which is desirable, for instance, in an analyzer formeasurement of blood gases, electrolytes or enzyme substrates, or acombination thereof.

The internal parameter must be measurable over the entire mixing rangeincluding the limiting values of the initial solutions, and thefunctional relationship between measured value of the internal parameterand mixing ratio of the working solution must be known.

The above method is particularly well suited for integration into acalibrating system for analyzers with a measuring chamber, whichmeasuring chamber is provided with a device for measurement of theconductivity of sample and calibrating solutions delivered into themeasuring chamber by means of a pump.

It is provided by the invention that the calibrating system comprise twocontainers with different initial solutions, where controlled valves areprovided for coarse mixing of the initial solutions to produce a workingsolution in a passage, and where the passage may be connected to thedevice for conductivity measurement, and where an evaluation unit isprovided for calculation of the exact mixing ratio of the initialsolutions from the value measured for conductivity.

Such a calibrating system is particularly useful in analyzers for themeasurement of blood gases, electrolytes and/or enzyme substrates.

Following is a more detailed description Of the mixing process asillustrated by examples and supported by FIGS. 1 to 4 of theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a mixing system employing a single-channel pump system,

FIG. 2 shows a mixing system employing a pressure cylinder system,

FIG. 3 shows a mixing system employing a multi-channel pump system,

FIG. 4 shows a mixing system for use in the calibrating system of ablood gas analyzer.

DETAILED DESCRIPTION OF THE INVENTION

The following procedures for coarse mixing (item A) and ratiomeasurement (item B) may be combined as required in a mixing system forimplementing the method of the invention.

A) Low-cost Methods for Coarse Mixing of Initial Solutions

In FIG. 1 two initial solutions in containers 1 and 1' are fedalternatingly into a common passage 3 by means of a single-channel pump2 to produce a working solution, the resulting mixture being homogenizedin passage 3. The mixing ratio is determined within certain limits viathe ratio of the opening times of valves 4, 4' in the suction lines 5,5' of the initial solutions.

In FIG. 2 fractions of the two initial solutions in cylinders 6, 6' aremetered and dispensed at a given pressure into a common passage 3, bymeans of valves 4, 4'. The mixing ratio is again predefined withincertain limits via the ratio of the opening times of valves 4, 4'.

In the variant of FIG. 3 the initial solutions are delivered from theircontainers 1, 1' in a given ratio via the pump channels of amulti-channel pump 7. By joining the lines 5, 5' into one common linebehind the pump 7 the initial solutions delivered are thus combined incorrespondence with the ratio of the channel delivery rates. In order topermit optional delivery of one of the two initial solutions in itsunmixed state into passage 3 for the purpose of calibration, bypasschannels 9, 9' with valves 8, 8' are provided.

B) Methods for Precise Determination of the Mixing Ratio

Generally speaking, determination of the mixing ratio must be moreaccurate than the required accuracy of the mixture. In addition, fordetermination of the mixing ratio an internal parameter must beevaluated in which the two initial solutions differ significantly. Themeasuring apparatus may be installed either at the site where themixture is processed or in the path towards the consumer of the workingsolution. Calibration of the mixing system is performed by measuring theunmixed initial solution.

FIG. 1 is concerned with ratio determination via conductivitymeasurement or measurement of electrical resistance, which may beperformed on initial solutions whose values for conductivity orelectrical resistance differ. As there is an (approximately linear)mathematical relationship between mixing ratio and conductivity orelectrical resistance, the actual mixing ratio may be accuratelycalculated from a measurement of conductivity or resistance by means ofa suitable device 10.

As is shown in FIG. 2, the optical transmission or absorption, which isdifferent for each initial solution, may be measured by a (schematicallydrawn) light detection unit 12 and used for determination of the mixingratio. In the light of one of the requirements of the invention, i.e.,no use of external markers, this method is suitable for use with initialsolutions whose colors are a priori different.

FIG. 4 shows a favorable application of the method in the calibratingsystem of an analyzer (not shown here) measuring blood gases,electrolytes or enzyme substrates, or a combination thereof, in which asingle-channel pump system 2 is combined with a device 10 forconductivity measurement (cf. FIG. 1).

For the calibration of such an analyzer solutions with specific gaspartial pressures are required. As such solutions have no storagestability they are prepared immediately before use by mixing two initialsolutions. It is not necessary to produce an accurate mixing ratio;knowledge of the actual mixing ratio obtained and thus theconcentrations and partial pressures of the substances required forcalibration will suffice.

For optimum integration into the analyzer the analyzer pump, which isconfigured as a single-channel pump 2, is connected to passage 3 of themixing system via a valve 11. The working mixture contained in passage 3upon mixing is drained via a connection 14 switched by a valve 13. Afterthey have passed the timed valves 4, 4' the initial solutions arecombined in a crosspiece 15. If necessary, passage 3 is vented by valve16.

If the system is integrated in an automatic analyzer as described, theaccurate mixing ratio to be measured in an evaluation unit 18 is bestdetermined by a conductivity measurement for the following reasons:

Contacts provided for conductivity measurement in measuring chamber 17may also be used for the guiding and positioning of samples;

Analysis of the working mixture is performed in a measuring chamber 17which is thermostat-controlled for several reasons;

In the above analyzer, the two initial solutions are characterized by asignificant difference in conductivity values, even if no specialprovisions are made.

The mixing ratio may therefore be accurately determined "on site", i.e.,where the working solution or the calibrating solution is actuallyprocessed in the measuring chamber.

An automatic analyzer for use with the system of the invention requiresonly the following additional elements: the mixer valves 4, 4', the twovalves 11 and 13 for connection of the device to the blood gas analyzer,and, optionally, a venting valve 16. The following elements are neededfor operation of the analyzer itself, their use in the system for mixingthe initial solutions being an additional application: the pump 2 andthe device 10 for conductivity measurement located in the sample ormeasuring chamber 17.

Another advantage of the calibrating procedure described above is thatany number of calibration points and calibration ranges are obtainablewith the use of the same initial solutions.

The calibration of automatic clinical analyzers is usually adjusted tophysiological standard values or expected values. By means of theprocess of the invention additional calibration points may easily beestablished outside the normal ranges. In the case of extremelypathological sample values, for instance, an additional calibratingpoint in the corresponding range would be useful for a correct analysis.

Some automatic analyzers are suitable for measuring identical parametersin different body fluids whose normal ranges of these parameters may becompletely different. An example would be electrolyte analysis usingion-selective electrodes in whole blood, serum, or plasma on the oneside, and in urine on the other side. In urine samples the expectedvalues for Na⁺, K⁺, and Cl⁻ have normal ranges which are completelydifferent from those in the other three sample types. Once again, theprocedure described above will permit calibrating values to be obtainedin accordance with the corresponding normal range of the sample withouthaving to exchange the initial solutions.

We claim:
 1. A method for providing a calibrating solution having anaccurately known value of a predetermined calibrating parameter fromfirst and second initial solutions which each exhibit a predeterminedinherent parameter but to a differing degree, said method consisting ofthe steps of (a) coarsely mixing said first and second initial solutionsat a miming ratio within a predetermined range to obtain a calibratingsolution, (b) measuring the value of said predetermined inherentparameter of said first and second initial solutions in said calibratingsolution to determine the exact ratio of said first and second initialsolutions in said calibrating solution, and (c) determining an exactvalue of said calibrating parameter of said calibrating solution fromsaid exact ratio of said first and second initial solutions in saidcalibrating solution determined in step (b).
 2. A method according toclaim 1, including the step of using said value for said inherentparameter determined in step (b) as a controlling or adjusting variablefor correcting said mixing ratio in step (a).
 3. A method according toclaim 1, wherein said calibrating parameter is at least one variablefrom a group consisting of pH value and CO₂ partial pressure of saidcalibrating solution.
 4. A method according to claim 1, wherein saidcalibrating parameter is at least one ionic concentration of a groupconsisting of Na⁺, K⁺, Li⁺, Ca⁺⁺, Mg⁺⁺, Cl⁻ and HCO₃ ⁻ of saidcalibrating solution.
 5. A method according to claim 3, wherein saidcalibrating parameter is at least one ionic concentration of a groupconsisting of Na⁺, K⁺, Li⁺, Ca⁺⁺, Mg⁺⁺, Cl⁻ and HCO₃ ⁻ of saidcalibrating solution.
 6. A method according to claim 1, wherein saidcalibrating parameter is at least one enzyme substrate concentration ofa group consisting of glucose, lactate, urea, and creatinine of saidcalibrating solution.
 7. A method according to claim 5, wherein saidcalibrating parameter is at least one enzyme substrate concentration ofa group consisting of glucose, lactate, urea, and creatinine of saidcalibrating solution.
 8. A method according to claim 1, wherein saidinherent parameter is conductivity or electrical resistance of saidcalibrating solution.
 9. A method according to claim 1, wherein saidinherent parameter is at least one of optical transmission, opticalabsorption, and intrinsic luminescence of said calibrating solution.